Optical sensing in the ultraviolet-visible (UV-vis) to the infrared (IR) spectrum is critical to a variety of activities, including: monitoring and remote sensing, fiber-optic communication, day/night-time surveillance, and medical imaging. Currently, to accomplish such sensing, separate sensors and/or materials are required to detect different sub-bands within the UV-vis to IR wavelength spectrum or range. For example, AlGaN, Si, InGaAs and PbS based photodetectors (PD) have been used to cover spectral responses that occur for wavelengths between 250 nm to 2600 nm. However, such PDs are required to be operated at low temperatures, such as 4.2 degrees Kelvin in the case of InGaAs based photodetectors for example, in order to reach high levels of detection sensitivity.
In order to avoid such low temperature operating requirements, tremendous attention has been given to the development of lead sulfide (PbS) quantum dots (QDs) based photodetectors (PDs), hereinafter referred to as PbS QD based PDs. For example, a responsitivity (R) of over 103 A/W with a detectivity (D*) of approximately 1013 Jones (1 Jones=1 cm Hz1/2W−1) has been achieved from PbS QD based PDs. In addition, a responsitivity (R) of approximately 107 A/W was further achieved from PbS QDs based PDs that incorporated highly electrically conductive graphene as light harvesters.
However, a planar thin film transistor (TFT) device structure was utilized in both cases, which led to the need for high driving voltages and slow response times, which was the result of large lateral electrode spacing (greater than 50 um) that was required for reducing the dark currents and for maintaining high gains in the TFTs. These limitations of such PbS QDs photodetectors substantially restrict their application in day/night-time surveillance and chemical/biological sensing, where high-speed and low power (e.g. low voltage) operation are desired.
In view of these drawbacks of current PbS QDs based photodetectors, it would be desirable to develop a PbS QDs based photodetector with a vertical device structure, where low driving voltages used. Unfortunately, the development of such PbS QDs has been limited due to the poor electrical conductivity of PbS QDs, which is caused by the occurrence of a substantial amount of trap states on the surface of the PbS QDs. In order to achieve high-performance PbS QDs based photodetector operation, the trap states in such photodetectors are required to be minimized or eliminated, as the trap states provide numerous sites for charge carrier recombination with the PbS QDs, while also leading to large leakage current, which results in low detectivity.
Therefore, there is a need for a photodetector that eliminates or reduces structural defects that cause trap states in the PbS QDs, by filling up the hard-to-access sites in the bandgap of the PbS QDs. In addition, there is a need for a photodetector that operates at reduced temperatures, such as room temperature, as well as low power, for optical detection of wavelengths in the spectrum between UV-vis (ultraviolet visible) to IR (infrared). There is also a need for a photodetector that has an operating performance that is similar to that of inorganic photodetectors, but which is capable of operation at room temperature. Additionally, there is a need for a photodetector that utilizes PbS QDs, which is configured with a vertical device structure.